Sensors facilitate the sensing of certain conditions within a patient. An implantable medical device (IMD) may include circuitry electrically coupled to one or more sensors to record patient conditions over time. For example, an implantable lead is typically used to couple a sensor to circuitry within an IMD housing.
As one example, electrochemical sensors are commonly used for diabetes patients, e.g., to monitor a patient's glucose levels. In that case, a lead can be implanted within a patient to deploy an electrochemical glucose sensor for sensing levels of glucose in the patient's blood stream. A distal end of the lead, which includes the electrochemical glucose sensor, may be positioned in the patient's blood stream, e.g., within a heart chamber. A proximal end of the lead may be coupled to circuitry within the IMD. The IMD may record the sensed conditions, which may be sent to an external programmer via telemetry, so that a physician can monitor the patient's blood glucose levels. Alternatively or additionally, the IMD may interpret the sensed conditions and possibly deliver therapy to the patient, e.g., by delivering doses of insulin or drug therapy to the patient if sensed glucose levels exceed one or more therapy thresholds.
Most glucose sensors use electrochemical methods such as an electroenzymatic method in which blood glucose is oxidized under glucose-oxidase control, producing gluconic acid and hydrogen peroxide. In that case, the electrochemical glucose sensor can be used to sense levels of gluconic acid and/or hydrogen peroxide in order to estimate a patient's glucose levels. Alternatively, the glucose levels may be determined or sensed more directly by the electrochemical glucose sensor. In any case, an electrochemical glucose sensor on a medical lead facilitates a chemical process which can be identified by electrical signals to measure a patient's glucose levels.
In addition to electrochemical glucose sensors a number of other types of electrochemical sensors have been developed to measure blood chemistry or the chemistry of other bodily fluids or material. In general, electrochemical sensors make use of one or more chemical processes and electrical signals can be used to measure conditions sensed by the chemical processes. Other types of sensors also exist including pressure sensors, optical sensors, and various other types of sensors.
Electrochemical sensors and many other types of sensors, however, typically suffer from stability and longevity problems. For example, the electrochemical process used by electrochemical glucose sensors can cause significant depletion of the glucose sensors over time, eventually causing the glucose sensor to be ineffective for monitoring levels of glucose in the patient's blood stream. Similarly, other sensors often have similar problems in that prolonged use causes depletion, wear or tissue overgrowth, which can undermine the sensing capabilities of the given sensor.
Sensor replacement is a major concern because replacement of conventional electrochemical sensor leads is typically a very invasive medical procedure in which a lead is removed from a patient's blood vessel, and a new lead is inserted to a high blood flow location, such as inside a heart chamber. In the replacement procedure, the new electrochemical sensor lead is typically inserted through the same blood vessel used for the previous electrochemical sensor lead. Removal of the lead can be very traumatic because of fibrosis around the lead. Moreover, repeated implantation of new leads can cause damage and scarring to the patient's blood vessel, making subsequent implantation more difficult and more dangerous to the patient. Infection is also a concern.